The invention relates in general to a polarization-maintaining optical fiber amplifier that is fabricated from non-polarization-maintaining gain optical fiber. More specifically, the invention employs coiling under tension to create a stress-induced axis of linear birefringence within the fiber.
A wide variety of applications in fiber optic photonics require the use of polarization-maintaining optical fibers. In such a fiber the polarization planes of linearly polarized light waves launched into the optical fiber are maintained during propagation with little or no cross coupling of optical power between the orthogonal polarization modes. In many of these systems there is also a need for polarization-maintaining optical fiber amplifiers. An optical fiber amplifier is a device that amplifies an optical signal directly, i.e., without the need to convert it to an electrical signal, amplify it electrically, and reconvert it to an optical signal.
An optical fiber amplifier uses an optical fiber having a rare-earth-doped core, which will be referred to hereinafter as a gain optical fiber. Although Er3+ is most commonly used as a rare-earth element in gain optical fibers, different rare earth elements such as Nd3+, Yb3+, Pr3+, Ho3+, Sm3+, and Tm3+ may be used. The rare-earth ion is optically excited, typically but not exclusively using the output of a diode laser; a signal beam propagating in the core experiences gain if a population inversion has been established by absorption of the pump beam by the rare-earth ions (and if the signal beam has a wavelength within the gain spectrum of the rare-earth dopant).
Optical fiber amplifiers generally out-perform conventional solid-state amplifiers in the following key areas: small-signal gain, tunability, beam quality (for single-mode optical fibers), immunity to mechanical and thermal disturbances, size, weight, cost, and electrical efficiency. One notable disadvantage of optical fiber amplifiers is their tendency to scramble the input polarization of the seed signal. This polarization-scrambling effect is a consequence of azimuthal asymmetry in the refractive-index distribution of the optical fiber, commonly referred to as optical fiber birefringence. A linearly polarized seed signal injected into the fiber will generally be converted to an unspecified, time-dependent elliptical polarization state, i.e., the fiber is not polarization maintaining.
In an ideal optical fiber having an azimuthally symmetric refractive-index profile, a signal injected into one end of the fiber will propagate through the optical fiber with its polarization state unchanged. Each of the transverse modes supported by the optical fiber waveguide can exist in two orthogonal polarizations (e.g., vertical and horizontal), and in a perfectly symmetric optical fiber these two polarization modes propagate at the same speed, independent of one another (i.e., the fiber is not birefringent). In practice, it is impossible to manufacture an optical fiber that has perfect azimuthal symmetry, and all real optical fibers thus exhibit non-zero birefringence. Core or cladding ellipticity and mechanical strain, which causes random refractive-index perturbations, are the main contributors to random birefringence in an optical fiber and thus to non-polarization-maintaining behavior.
There are several solutions for the problem of polarization scrambling due to random birefringence in an optical fiber. As described below, random birefringence may be corrected by utilizing a polarization controller, a Faraday mirror, or a polarization-maintaining optical fiber.
The simplest solution to the problem of polarization scrambling due to optical fiber birefringence is the use of a polarization controller. There are a number of different designs for polarization controllers, but in all cases the principle of operation is the same. The birefringent optical fiber is sandwiched between two waveplates whose orientation and retardation are independently adjustable; alternatively the fiber input or output beam may be directed through three waveplates whose orientations but not retardations are adjustable. It can be shown that for any fiber birefringence, it is always possible to set the adjustable waveplates such that there is no change in the polarization state for a signal passing through the entire system (fiber plus polarization controller). Unfortunately, the birefringence properties of the optical fiber are sensitive to environmental factors, such as changes in temperature or mechanical disturbances. Changes in the birefringence properties of the optical fiber over time necessitate readjustment of the polarization controller, making it unsuitable for most real-world applications.
In some optical fiber circuits, a device known as a Faraday mirror can be used to compensate for optical fiber birefringence. In a Faraday mirror, a signal passing through the birefringent optical fiber must retrace it""s path through the optical fiber, traveling in the opposite direction on the return trip, thereby creating a folded optical path. The Faraday mirror never needs adjustment and is able to compensate for rapid changes in birefringence, limited only by the round-trip propagation in the fiber. The main disadvantage of the Faraday mirror is that it is applicable to only a small subset of optical fiber circuits and is therefore lacking in generality. In addition, commercially available Faraday mirrors suffer from one or more the following drawbacks: high cost, large size and weight, limited power-handling capability, and limited wavelength range (i.e., a wavelength range smaller than the range over which the fiber exhibits gain).
The best all-around solution to the problem of optical fiber birefringence is the use of a polarization-maintaining (PM) optical fiber. In a PM optical fiber, a very large azimuthal asymmetry is introduced intentionally during the manufacturing process. The goal is to create a controlled linear birefringence that is very large (compared with the random birefringence) and oriented along a well-defined axis. This birefringence can be generated by fabricating an optical fiber core with an elliptical cross-section, by subjecting an optical fiber to mechanical stress, or by a combination of both techniques. When this linear birefringence is much greater than the random birefringence due to optical fiber imperfections, good PM behavior is obtained.
FIGS. 1-4 show cross-sections of various conventional PM optical fibers 10, looking down the optical fiber axes. The components of the PM optical fibers may include a core 20, a cladding 22, and stress elements 24.
The stress elements 24 shown in FIGS. 1-3 for the bow-tie 12, Panda 14, and oval-inner-clad 16 PM optical fibers are fabricated from a glass whose thermal expansion coefficient is different (usually greater) than that of the cladding 22 glass, which is usually silica. During manufacture, the optical fiber 10 is drawn from molten glass and therefore starts out stress-free. Solidification occurs several hundred degrees above room temperature, at which point the optical fiber 10 is capable of accumulating mechanical stress. As the optical fiber 10 cools further, the stress elements 24 contract differently (usually more) than the surrounding cladding, generating a stress field that is azimuthally asymmetric. Specifically, the stress distribution has two-fold bilateral symmetry, in which the mirror planes of minimum and maximum stress are perpendicular to each other. The stress-induced change in the refractive index has these same symmetry properties. Within each PM optical fiber 10 there is thus a fast axis 26 and a slow axis 28 that are mutually perpendicular (analogous to a waveplate). Because of the difference in index of refraction, a ray of light whose polarization direction is aligned along the fast axis propagates at a slightly faster speed than a ray of light whose polarization direction is aligned along the slow axis.
If linearly polarized light is injected into a PM optical fiber with its polarization direction aligned parallel to either the fast or slow axis of linear birefringence, no polarization scrambling is observed. There are three factors that determine how well such an optical fiber will preserve polarization in practice: the amount of stress-induced linear birefringence, the amount of random birefringence due to fiber imperfections, and the length of fiber. As mentioned earlier, good PM behavior is obtained if the induced linear birefringence is made much greater than the random birefringence due to optical fiber imperfections. In a well-designed PM fiber, random birefringence is thus kept to a minimum while the linear birefringence is made as large possible. A typical value for commercially available PM optical fiber is xcex94n=2xc3x9710xe2x88x924, where xcex94n is the difference between the refractive indices of the fast and slow axes.
There has been very little progress in the area of optical fiber amplifiers based on PM rare-earth-doped gain optical fiber. The existing methods for gain optical fiber fabrication include modified chemical vapor deposition and solution doping, and these methods are generally incapable of producing optical fiber preforms with anything other than an azimuthally symmetric distribution of constituents. The incorporation of stress rods, for example, is not straightforward. At present there is only one rare-earth-doped PM gain optical fiber that is commercially available, an Er-doped gain optical fiber manufactured by Fibercore Ltd. (UK).
The Fibercore Er-doped gain optical fiber is not appropriate for the construction of high-power optical fiber amplifiers because it is single-clad and can thus be efficiently pumped only with single-mode pump sources. Single-mode pump sources for single-clad gain optical fiber are costly and provide relatively low pump powers (about $3000 for a 0.1 W pump source). It is therefore preferable to use a cladding-pumped amplifier with a double-clad gain optical fiber. The advantage of using double-clad gain optical fiber is that low-brightness, broad-area (multimode), laser diode pump sources can be efficiently coupled into the inner cladding. These multimode sources offer much higher power at much lower cost (less than $300 per Watt of pump power) than single-mode pump sources. In addition, the optics used to couple pump light into the double-clad gain optical fiber is much less sensitive to misalignment by mechanical disturbances or fluctuations in ambient temperature. This combination of properties makes double-clad gain optical fibers well suited to a wide variety of optical fiber amplifier applications. Unfortunately, PM gain optical fibers incorporating rare-earth dopants other than Er3+ and double-clad PM gain optical fibers are not commercially available. A PM, Yb-doped, double-clad optical fiber was recently demonstrated, but this fiber is not yet available commercially.
It would therefore be desirable to provide a PM optical fiber amplifier that uses non-polarization-maintaining gain optical fiber, which is widely available.
These and other objects of the invention are achieved in a preferred method of the invention.
An aspect of the present invention is a method of forming a linear polarization-maintaining optical fiber for use in an amplifier, the method comprising the steps of: providing a rare-earth-doped non-polarization-maintaining optical fiber having one or more cladding layers and having a random birefringence; providing a mandrel having a selected diameter; coiling the non-polarization-maintaining optical fiber under a selected tension around the mandrel to induce a linear birefringence greater than the random birefringence in the non-polarization-maintaining optical fiber thereby forming a polarization-maintaining optical fiber; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Ho3+, Er3+, Sm3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the tension is chosen to avoid undesirable weakening of the non-polarization-maintaining fiber.
Another aspect of the present invention is an optical amplifier with a rare-earth-doped polarization-maintaining optical fiber, the amplifier comprising: a mandrel having a selected diameter; a rare-earth-doped polarization-maintaining optical fiber having one or more cladding layers and having linear birefringence greater than a random birefringence; and a pump source which is coupled to the optical fiber; wherein the rare-earth-doped polarization-maintaining optical fiber is coiled around the mandrel under tension; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Ho3+, Er3+, Sm3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the tension is chosen to avoid undesirable weakening of the polarization-maintaining fiber.
A further aspect of the present is a method of forming a linear polarization-maintaining optical fiber amplifier, the method comprising the steps of: providing a rare-earth-doped non-polarization-maintaining optical fiber having one or more cladding layers and having a random birefringence; providing a mandrel having a selected diameter; coiling the non-polarization-maintaining optical fiber under a selected tension around the mandrel to induce a linear birefringence greater than the random birefringence in the non-polarization-maintaining optical fiber thereby forming a polarization-maintaining optical fiber; and coupling a pump source to polarization-maintaining optical fiber; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Er3+, Sm3+, Ho3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the tension is chosen to avoid undesirable weakening of the non-polarization maintaining fiber.
A further aspect of the present invention is an optical amplifier with a rare-earth-doped polarization-maintaining optical fiber, the amplifier comprising: a mandrel having a selected diameter; a rare-earth-doped polarization-maintaining optical fiber having one or more cladding layers and having linear birefringence greater than a random birefringence; and a pump signal source which is coupled to the optical fiber; wherein the rare-earth-doped polarization-maintaining optical fiber is coiled around the mandrel; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Ho3+, Er3+, Sm3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the diameter of said polarization-maintaining optical fiber is from about 50 xcexcm to about 1000 xcexcm.
Another aspect of the present invention is a method of forming a linear polarization-maintaining optical fiber for use in an amplifier, the method comprising the steps of: providing a rare-earth-doped non-polarization-maintaining optical fiber having one or more cladding layers, a diameter and a random birefringence; providing a mandrel having a selected diameter; coiling the non-polarization-maintaining optical fiber around the mandrel to induce a linear birefringence greater than the random birefringence in the non-polarization-maintaining optical fiber thereby forming a polarization-maintaining optical fiber; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Ho3+, Er3+, Sm3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the diameter of the non-polarization-maintaining optical fiber is from about 50 xcexcm to about 1000 xcexcm.
A further aspect of the present invention is a method of forming a linear polarization-maintaining optical amplifier for use in an amplifier, the method comprising the steps of: providing a rare-earth-doped non-polarization-maintaining optical fiber having one or more cladding layers, a diameter and a random birefringence; providing a mandrel having a selected diameter; coiling the non-polarization-maintaining optical fiber under a selected tension around the mandrel to induce a linear birefringence greater than the random birefringence in the non-polarization-maintaining optical fiber thereby forming a polarization-maintaining optical fiber; coupling a pump source to polarization-maintaining optical fiber; wherein the mandrel diameter is chosen to avoid significant bend loss; wherein the rare-earth dopant is selected from the group consisting Nd3+, Yb3+, Pr3+, Er3+, Sm3+, Ho3+ and Tm3+; wherein the mandrel diameter is selected to be from about 0.1 cm to about 10 cm; and wherein the non-polarization-maintaining optical fiber diameter is from about 50 xcexcm to about 1000 xcexcm.